The present invention relates to an apparatus for the conversion of waste, including the processing, treatment or disposal of waste. In particular, the present invention is directed to a system and method for decongesting a furnace in a plasma torch based waste processing plant.
The processing of waste including municipal waste, medical waste, toxic and radioactive waste by means of plasma-torch based waste processing plants is well known. Referring to FIG. 1, a typical prior art plasma-based processing plant (1) comprises a processing chamber (10) typically in the form of a vertical shaft, in which typically solid, and also mixed (i.e., generally, solid plus liquid and/or semiliquid), waste (20) is introduced at the upper end thereof via a waste inlet means comprising an air lock arrangement (30). One or a plurality of plasma torches (40) at the lower end of the chamber (10) heats the column (35) of waste in the chamber (10), converting the waste into gases that are channeled off via outlet (50), and a liquid material (38) (typically molten metals and/or slag) which is periodically or continuously collected at the lower end of the chamber (10) via reservoir (60). Oxidising fluid, such as air, oxygen or steam (70) may be provided at the lower end of the chamber (10) to convert carbon, produced in the processing of organic waste, into useful gases such as CO and H2, for example. A similar arrangement for dealing with solid waste is described in U.S. Pat. No. 5,143,000, the contents of which are incorporated herein by reference thereto.
Two problems commonly encountered that prevent smooth operation of such processing plants or furnaces are:xe2x80x94
(One) Bridging.
(Two) Unprocessed solid deposition.
The bridging phenomenon relates to a blockage that occurs as a result of the passage of solid material through a channel such as the chamber (10), the problem being further exacerbated when some of the solids liquefy. Many organic materials that may be found in the waste column (35) undergo a number of transformations during processing in the chamber (10). These transformations include, as a function of increasing temperature, the formation of gas products, the formation of liquid and semi-liquid pitch or bitumen, the evaporation of the pitch and charcoal or coke formation at high temperatures. These transformations may be occurring simultaneously at different parts of the furnace due to the temperature profile in the chamber (10). Thus, while raw or unprocessed waste may be found at the upper end of the waste column (35), the organic materials are converted to charcoal at the bottom end of the waste column (35), and to bitumen in a central portion of the waste column (35).
During the bitumnisation process of the organic waste, several pieces of bituminised waste may coalesce to form a full or partial bridge blockage in the furnace, as illustrated at (A) in FIG. 1.
Inorganic waste is normally dealt with at the lower, hotter parts of the chamber (10). Because of the non-homogenous composition of the waste and the temperature profile within the chamber (10), some inorganic waste may melt at higher portions of the chamber (10), and flow downwards, adhering to other waste and in some cases causing several pieces of waste to adhere to one another, resulting in a blockage. In fact, the molten waste may adhere to the walls of the chamber (10) and even crystalise there if the wall temperature is lower than the melting point of the waste, also leading to a bridge-type phenomenon within the chamber (10).
Another type of bridging phenomenon may occur as a direct result of the passage of solid waste through the furnacexe2x80x94a bridge-type formation, similar to a vaulted ceiling in form, can occur naturally within the refuse column, particular when the refuse is in granulated form, as illustrated at (B) in FIG. 1. The bridge-type formation provides a stable load bearing structure for the column of refuse, redirecting the weight of the column from the centre thereof to the edges in contact with the walls of the chamber (10), thereby preventing the flow of refuse via gravity through the furnace. The presence of a bridging phenomenon within the chamber (10) results in a reduction or total stoppage of the feed rate of waste through chamber (10).
Japanese Patent Application No. 10019221A2 addresses a bridging phenomenon problem by providing a number of mechanical devices which are inserted into the column of refuse from the sides or from the top of the furnace. These devices provide an external mechanical force to the waste in a direction towards the inside of the furnace, accomplished by either rotating members or axially displaceable members. While possibly effective in some cases, the mechanical devices are subject to a great deal of wear and tear and to high thermal stresses, and need to be replaced or serviced fairly frequently. Further, when not needed, the devices actually represent a partial blockage with respect to the column. The devices are also able to directly apply force in relatively isolated points within the furnace. Furthermore, incorporation of such mechanical devices in a furnace made from refractory material is not straightforward. Japanese Patent Publication Nos. JP 10 110917 and JP 10 089645 each describe a vertical melting furnace which is externally bulged to form a combustion space, thereby enabling continuous waste disposal. While these two patents are directed towards the prevention of bridging, they are not particularly effective in this regard, nor do not provide any solution to removing a bridging phenomena or for reducing propagation thereof
French Patent No. 2,708,217 describes a plasma-torch based system in which the plasma arc is permanently submerged between the liquid products and the torch, within a reaction zone of the material being treated. Japanese Patent Application No. 05346218 describes a waste melting furnace in which a waste feed device, and air feed pipe and an auxiliary fuel feed device are provided to monitor and control melting conditions of the waste in order to minimise consumption of the auxiliary fuel. U.S. Pat. No. 4,831,944 describes another type of furnace wherein the plasma jets are inclined with respect to the corresponding radius of the column. U.S. Pat. No. 4,848,250 is directed to an apparatus and method for converting refuse to thermal energy, metal and slag devoid of particulate material. However, none of these references is directed to the problem of bridging, nor do they provide a solution therefore, less so in the manner of the present invention.
Waste material may comprise many different substances, some of which may have very high melting temperatures. Such substances may include, for example, refractory bricks, some types of rocks and stones, and also aluminium oxide (Al2O3). Furthermore, the waste may also contain products having a high aluminium content, and the aluminium may be oxidised to aluminium oxide by the hot oxidising means provided at the lower end of the chamber (10). The melting temperature for aluminium oxide is about 2050xc2x0 C., and the melting point for other oxides that may also be found or formed within the waste column (35) include for example about 2825xc2x0 C. for Magnesium oxide (MgO), and about 2630xc2x0 C. for calcium oxide (CaO). However, the temperature at the lower end of the chamber (10), i.e., of the liquid material (38) is in the order of between about 1500xc2x0 C. and about 1650xc2x0 C. Thus, unprocessed solid deposition occurs when certain types of solid waste having a high melting temperature, or when some substances are transformed into oxides having a high melting temperature, rather than liquefy persist in a solid state during the normal operation of the furnace. The deposition of such solids at the lower end of the chamber (10) leads to blockage thereat, preventing run-off of liquid material (38) (typically molten metals and/or slag) to reservoir (60), as illustrated at (C) in FIG. 1. The same problem may occur when the viscosity of molten material is increased significantly due to a change in its composition. Thus, while this problem does not directly affect the feed rate of the waste through chamber (10), the flow rate of the liquid material (38) may be drastically reduced or stopped, which indirectly results in some reduction in the flow rate of refuse through the chamber (10). In the art, such xe2x80x9cunprocessed solidsxe2x80x9d need to be treated with fluxing agent, which enable the solids to dissolve therein, forming solutions with relatively lower crystallisation temperature and of lower viscosity than the unprocessed solids may have in the liquid state. The resulting solutions are subsequently melted and removed from the lower part of chamber (10) in the normal manner. For example, Calcium Oxide (CaO), and Aluminium Oxide (Al2O3) each have relatively high individual melting points. However, if mixed together with quartz (Silicon Oxide (SiO2)) in appropriate proportions (e.g., SiO2-62%, CaO-23.25%, A2O3-14.75%), the resulting mixture begins to melt at about 1165xc2x0 C., and liquid droplets begin to form at about 1450xc2x0 C., which is well within the temperature range existing at the lower end of the chamber (10). Similarly, while the existence of quartz (SiO2) or
Aluminium Oxide (Al2O3) each increase viscosity and thus decrease the fluidity of liquid material (38), the addition of fluxing agents such as CaO, MgO, MnO, FeO serve to decrease viscosity of the liquid material (38) and thus to promote run-off thereof in some cases, Aluminium Oxide can act as a fluxing agent, the addition of small quantities thereof to slag containing large amounts of CaO having the effect of lowering the viscosity of the mixture. Unprocessed solids may be dissolved in liquid slag if in contact therewith, since the liquid slag comprises many different compounds in a dissociated state, enabling many different crystal compositions to be formed at different temperatures. The dissolving process is accelerated if the viscosity and surface tension of the melt are low, and these parameters will depend on the composition of the solids as well as of the melt, and on the temperature of the melt. It is also known that raising the temperature of the slag also serves to reduce its viscosity.
In the prior art, if and when it is determined that solid deposition has occurred, fluxing agents are then provided at the top end of the chamber (10) (typically manually) at the waste inlet means of the apparatus, which is somewhat ineffective since the agents have to percolate through the whole column of refuse, or at least pass together with the refuse to the lower part of the chamber, which takes a lot of time. If there is also bridging within the chamber (10), the fluxing agents cannot be applied to the solids, and thus the furnace has to be shut down, the refuse removed from the chamber and the bridging destroyed manually, before the solids can be accessed. Of course, by then all of the slag at the lower end of the chamber (10) has also solidified. In order to address either bridging or solid deposition within the processing chamber of a plant, the first step is to identify the presence thereof This not a simple matter, and is in fact significantly complicated in many instances by other factors.
For example, one indicator of the presence of bridging and/or of solid deposition is a decrease in the flow rate of waste through the processing chamber. However, as explained in more detail below, the changing composition of the waste itself may also affect the waste flow rate.
The composition of waste provided to the processing chamber may vary tremendously over any given time period, and may include any relative proportions of organic to inorganic waste, and any relative proportion of liquids to solids. While organic waste is converted to product gases (using oxygen-containing reagents), inorganic waste needs to be melted to a liquid, whose viscosity will depend on the constitution of the inorganic waste and the temperature thereof. Thus, if the waste that is fed to the processing chamber comprises a high proportion of inorganic material, there may be a decrease in the flow rate of waste through the chamber and/or solid deposition, simply because the primary plasma torches cannot deal with the large quantity of inorganic waste quickly enough. It is not generally possible to measure the concentration of some of the inorganic components of the wastexe2x80x94such as stones and glass, for examplexe2x80x94and usually visual monitoring of the waste by the plant operators is the only way of providing any estimate regarding the composition of any batch of waste being fed to the plant. When it is determined that the waste comprises a high degree of inorganic waste, then either the waste needs to be diluted with organic waste, or the feed rate to the processing chamber needs to be reduced.
On the other hand, a different problem is encountered when the waste comprises high levels of organic waste. Here, carbon in the form of coke or charcoal is produced at higher than normal amounts after drying and pyrolysis of the waste. Correspondingly, greater amounts of oxidising agents must be provided to convert the carbon to product gases. If the oxidising agents include steam, then more power is needed to be provided to the chamber since steam reacts with carbon endothermically. Unless more oxidising agents are provided together with greater power by the primary plasma torches, the flow rate of waste through the processing chamber will decrease, and it will then difficult to determine if the lowering in the waste flow rate is as a result of bridging or of coke build-up.
Thus, the waste flow rate through the processing chamber is not only affected by the presence of bridging and/or solid deposition, but also by the actual composition of the waste.
Another indication that there is solid deposition may be provided by an increase in the level of liquid product within the chamber. However, high viscosity of inorganic liquids at the lower end of the chamber also results in a slower rate of flow of liquid product, which in turn leads to a rise in the level thereof. It is not normally possible to determine whether the cause of a rising level of liquid product is solid deposition, or the high viscosity of the liquid product, or a mixture of the two. In any case, as in the case of solid deposition, fluxing agents as well as additional power to the chamber may help to lower the viscosity of the liquid product and thus provide a solution when this problem is encountered.
It is therefore an aim of the present invention to provide a system for dealing with bridging phenomena which overcomes the limitations of prior art systems and methods.
It is another aim of the present invention to provide such a system incorporated as an integral part of a plasma-torch based type mixed waste converter.
It is another aim of the present invention to provide a second system for dealing with unprocessed solids directly in a plasma-torch type processing apparatus.
It is another aim of the present invention to provide such a second system that incorporates a fluxing agent feed system for feeding fluxing agent directly into a plasma-torch type processing apparatus.
It is another aim of the present invention to provide such system that are relatively simple mechanically and thus economic to produce as well as to maintain.
It is another aim of the present invention to provide a method for operating a plasma-based waste processing plant such as to minimise blockages therein due to bridging and/or unprocessed solids.
The present invention achieves these and other aims by providing at least one and preferably a plurality of auxiliary plasma torches at strategic locations within the chamber (10) and directed towards the waste column. When a bridge forms within the chamber (10) one or more auxiliary plasma torches may be operated such as to provide an additional heat source where needed. This heat source serves to quickly heat the organic solids and thus pass through the bituminsation stage and to the charcoal formation as quickly as possible. The additional heat source may be in the neighborhood of the bridge, but may also be near the bottom end of the chamber (10). In the latter case, the additional temperature at the bottom of the chamber (10) effectively moves the combustion and gasification zones for the charcoal to a higher part of the chamber, altering the temperature profile. This helps to pass the bituminisation stage quickly, and effectively destroys such bridges. The heat source also enables the inorganic wastes to be heated rapidly to pass beyond the melting stage relatively quickly. The debridging process may be further enhanced by providing secondary plasma torches at various levels upwards of the primary torches, the secondary torches at any level being operated as and when needed to achieve the desired effect. Further, the heat source also enables a thermal shock front to be directed at the bridge, disrupting and/or destroying and/or melting the bridge, which is also useful for dealing with bridge-type phenomena which occur naturally due to the flow of solids along the chamber (10). The chamber may also be provided with at least one fluxing agent inlet at the lower part of the chamber such as to enable appropriate fluxing agents to be directly applied to the deposited xe2x80x9cunprocessed solidsxe2x80x9d and/or to liquid products of high viscosity.
The present invention is directed to a system for maintaining substantially decongested a waste converting apparatus, the waste converting apparatus having a waste converting chamber adapted for accommodating a column of waste, at least one primary plasma torch means for generating a hot gas jet at an output end thereof and for directing said jet towards a bottom longitudinal part of the chamber, said system comprising:xe2x80x94
at least one waste flow rate sensing means at least for detecting a first predetermined status of a flow rate of waste in said chamber;
at least one liquid product level sensing means at least for detecting a second predetermined status of a liquid product level in said chamber;
at least one secondary plasma torch means having an outlet in said chamber such that during operation of said system a high temperature zone may be selectively provided within said converting chamber for at least partially removing a bridge-type congestion from said chamber and/or to substantially prevent occurrence or propagation of such a congestion;
said secondary plasma torch means being selectively operable at least in response to said predetermined first status and said predetermined second status being detected.
The secondary plasma torch means may be located intermediate between said primary plasma torch means and an upper portion of said chamber. The said waste converting apparatus may comprise at least one gas outlet means at an upper longitudinal part of the chamber, and at least one said secondary plasma torch means may be located within a lower third of the said chamber taken vertically between said primary plasma torch means and said gas outlet means. Additionally or alternatively, at least one said secondary plasma torch means is located within a middle third of the said chamber taken vertically between said primary plasma torch means and said gas outlet means.
The first predetermined status may correspond to a detected waste flow rate lower than a predetermined minimum; the said second predetermined status corresponds to a detected liquid product level not greater than a predetermined maximum.
The present invention also relates to an apparatus for converting waste comprising:xe2x80x94
a waste converting chamber adapted for accommodating a column of waste, said chamber having an upper end;
at least one primary plasma torch means for generating a hot gas jet at an output end thereof and for directing said jet towards a bottom longitudinal part of the chamber;
at least one liquid product outlet means at a lower longitudinal part of said chamber;
said apparatus further comprising a first decongestion system for decongesting waste within said waste converting apparatus, said first system comprising:xe2x80x94
at least one waste flow rate sensing means at least for detecting a first predetermined status of a flow rate of waste in said chamber;
at least one liquid product level sensing means at least for detecting a second predetermined status, of a liquid product level in said chamber;
at least one secondary plasma torch means having an outlet in said chamber such that during operation of said system a high temperature zone may be selectively provided within said converting chamber for at least partially removing a bridge-type congestion from said chamber and/or to substantially prevent occurrence or propagation of such a congestion;
said secondary plasma torch means being selectively operable at least in response to said predetermined first status and said predetermined second status being detected.
Preferably, the at least one secondary plasma torch means is located intermediate between said primary plasma torch means and said upper end of said chamber.
The apparatus typically at least one gas outlet means at an upper longitudinal part of the chamber, and at least one said secondary plasma torch means is located within a lower third of the said chamber taken vertically between said primary plasma torch means and said gas outlet means. Optionally, at least one said secondary plasma torch means is located within a middle third of the said chamber taken vertically between said primary plasma torch means and said gas outlet means.
The first predetermined status typically corresponds to a detected waste flow rate lower than a predetermined minimum, while the second predetermined status typically corresponds to a detected liquid product level not greater than a predetermined maximum.
Preferably, the apparatus comprises a plurality of said second plasma torch means. At least some of said plurality of said second plasma torch means may distributed longitudinally and/or circumferentially with respect to said chamber.
The apparatus may further comprise at least one, and preferably a plurality of application points adapted for selectively enabling introduction of a plasma torch means with respect to said chamber. Each said application point typically comprises a suitable sleeve for accommodating therein a said second plasma torch such that during operation of said second plasma torch a high temperature zone provided inside the chamber at a predetermined location correlated to said corresponding application point, and wherein said sleeve is selectively sealable to prevent communication between the chamber and the outside when said sleeve is not accommodating a said second plasma torch. At least some of said plurality of said application points may be distributed longitudinally and/or circumferentially with respect to said chamber.
The apparatus preferably further comprises suitable control means for controlling operation of said first decongestion system operative connected to the waste flow rate sensing means, the liquid product level sensing means and the secondary plasma torch means. The apparatus preferably also comprises at least one suitable gas flow rate sensing means for monitoring the volume flow rate of product gases provided by said apparatus via said gas outlet means, and control means is operatively connected to said gas flow rate sensing means.
The apparatus typically further comprises waste inlet means associated with said upper part of said chamber. The waste input means may comprise an air lock means comprising a loading chamber for isolating a predetermined quantity of said waste sequentially from an inside of said chamber and from an outside of said chamber.
The apparatus may also further comprise waste composition determination means for at least partially determining a composition of waste fed to the said chamber, and the waste composition determination means is operatively connected to said control means.
The apparatus may further optionally comprise a second decongestion system for decongesting waste within said waste converting apparatus, said system comprising:xe2x80x94
at least one fluxing agent inlet means in said chamber separate from said waste inlet means, for selectively providing at least a quantity of at least one fluxing agent to a lower part of said chamber for at least partially removing a solid deposition type congestion and/or high viscosity liquid product-type congestion from said chamber, and/or to substantially prevent occurrence or propagation of such a congestion;
at least one said liquid product level sensing means at least for detecting a third predetermined status of a liquid product level in said chamber;
said at least one fluxing agent inlet means being selectively operable at least in response to said predetermined third status being detected.
Typically, the third predetermined status corresponds to a detected liquid product level substantially greater than a predetermined maximum, and the liquid level sensing means is adapted for selectively detecting said second status or said third status of liquid product level. At least one fluxing agent inlet means is located intermediate between said at least one liquid products outlet means and said waste inlet means. At least one fluxing agent inlet means may be located intermediate between said primary plasma torch means and said waste inlet means. Optionally, the fluxing agent inlet means may be vertically spaced from said primary plasma torch means by a predetermined spacing such as to enable a fluxing agent provided to said chamber via said fluxing agent inlet means to be substantially melted by means of said primary torch means. The fluxing agent inlet means is operatively connected to at least one suitable source of fluxing agent.
The second decongestion system advantageously also comprises at least one secondary plasma torch means having an outlet in said chamber such that during operation of said system a high temperature zone may be selectively provided within said converting chamber such as to enable a fluxing agent provided to said chamber via said fluxing agent inlet means to be substantially melted by means of said secondary torch means. Optionally, the fluxing agent inlet means and the second plasma torch means are disposed in a mixing chamber in communication with said chamber.
The fluxing agent may be provided in powdered form or in granulated form, and may include SiO2 (or sand), CaO (or CaCO3), MgO, Fe2O3, K2O, Na2O, CaF2, borax, dolomite, or any other suitable fluxing material including any suitable composition comprising at least one suitable fluxing material.
The present invention is also directed to a method for decongesting an apparatus for converting waste, wherein said apparatus comprises
a waste converting chamber adapted for accommodating a column of waste;
at least one primary plasma torch means for generating a hot gas jet at an output end thereof and for directing said jet towards a lower longitudinal part of the chamber;
at least one liquid product outlet means at a lower longitudinal part of said chamber;
wherein said method comprises:xe2x80x94
(a) providing at least one secondary plasma torch means having an outlet in said chamber such that during operation of said system a high temperature zone may be selectively provided within said converting chamber for at least partially removing a bridge-type congestion from said chamber and/or to substantially prevent occurrence or propagation of such a congestion;
(b) monitoring the flow rate of waste within said chamber via suitable waste flow rate sensing means;
(c) monitoring the level of liquid products at a lower longitudinal part of said apparatus via suitable liquid product level sensing means;
(d) if the volume flow rate at (b) decreases below a predetermined minimum and the level at (c) does not substantially increase above a predetermined maximum value, operating at least one said second plasma torch means;
(e) maintaining operation of said secondary plasma torch means until the waste flow rate at (b) is substantially restored to its predetermined minimum or until the level at (c) is substantially restored to its predetermined maximum, whereupon steps (b) to (e) are repeated.
Optionally, at least one said secondary plasma torch in step (a) may be provided at a lower portion of said chamber and at least one other said secondary plasma torch is provided at an upper part of said chamber with respect to said lower portion, and wherein steps (d) and (e) are replaced with the following steps:xe2x80x94
(f) if the volume flow rate at (b) decreases below a predetermined minimum and the level at (c) does not substantially increase above a predetermined maximum value, operating at least one said second plasma torch means at said lower end of said chamber according to a first operating mode;
(g) if the volume flow rate at (b) is still below said predetermined minimum and the level at (c) has not substantially increased above said predetermined maximum value, operating at least one said second plasma torch means at said upper part of said chamber;
(h) maintaining operation of said secondary plasma torch means at the upper part of said chamber until the waste flow rate at (b) is substantially restored to its predetermined minimum or until the level at (c) is substantially restored to its predetermined maximum, whereupon steps (b), (c), (f), (g) and (h) are repeated.
The first operating mode may comprise activating the said at least one secondary plasma torch at said lower end of said chamber for a predetermined time interval and then deactivating the same.
The method may further comprise providing the apparatus with at least one fluxing agent inlet means in said chamber separate from said waste inlet means, for selectively providing at least a quantity of at least one fluxing agent to a lower part of said chamber for at least partially removing a solid deposition type congestion and/or high viscosity liquid product-type congestion from said chamber, and/or to substantially prevent occurrence or propagation of such a congestion, said method further comprising the steps;
(i) if the level at (c) substantially increases above said predetermined maximum value, operating at least one said second plasma torch means at said lower end of said chamber according to a second operating mode;
(j) if the level at (c) has not decreased to at least said predetermined maximum value, operating at least one said second plasma torch means at said upper part of said chamber;
(k) providing a predetermined quantity of at least one fluxing agent to chamber via said fluxing agent inlet means until the level at (c) is substantially restored to its predetermined maximum, whereupon steps (b), (c), (i), (0) and (k) are repeated.
Preferably, the second operating mode comprises activating the said at least one secondary plasma torch at said lower end of said chamber for a predetermined time interval and then deactivating the same.